Epoxy resin is superior in heat resistance, adhesiveness, water resistance, mechanical strength and electrical properties, and is used in semiconductor sealing resin, laminate resin, coating material, adhesive, ink and various sealing materials. A method of curing the epoxy resin includes a method of using a curing agent such as an acid anhydride, amine compound, isocyanate compound and phenolic compound and polymerization reaction (addition reaction) of epoxy resin itself, that is, a photo/thermosetting system by cation polymerization.
As the curing system using cation polymerization, the cation polymerization which involves using not only the epoxy resin but also oxetane resin, vinyl ethers and styrene or derivatives thereof is known, and particularly the cation polymerization using epoxy resin and oxetane resin is extensively examined.
Among optical recording media, DVD (digital video disk or digital versatile disk) capable of recording large-capacity information attracts attention in recent years, and techniques wherein the memory capacity of one side of an optical disk is increased to 20 to 30 G bytes by utilizing a blue-violet laser light source at a wavelength of 405 nm and reducing a protective layer covering the recording layer to 0.1 to 0.3 mm are proposed (Japanese Journal Applied Physics, Vol. 30 (2000)). This DVD, unlike a conventional structure wherein two disks are attached, is composed fundamentally of one disk, and thus requires a surface protective layer. With respect to a material of this protective layer, Japanese Journal Applied Physics, Vol. 39, 775-778 (2000) shows (i) a method that involves spin-coating a photo cation polymerizable resin composition onto an optical disk and then irradiating it with UV rays in a nitrogen atmosphere to form a protective layer and (ii) a method that involves spin-coating a photo cation polymerizable resin composition onto a polycarbonate sheet, attaching it to an optical disk and irradiating it with UV rays to form a protective layer.
The performance required by these protecting coatings is that for recording with a blue-violet laser at 405 nm as a light source, the protective coating has excellent light transmittance at 405 nm and specifically requires 90% light transmittance. It is also desired that after spin coating or attachment of a polycarbonate sheet, the base polycarbonate plate is not deformed by curing upon irradiation with UV rays, and it is thus desired that the cure shrinkage of the surface protective material for optical disk is low. From this viewpoint, photo cation polymerization is preferably used in ring-opening polymerization. Further, there is a high degree of freedom in design of hardness by regulation of crosslinking density.
However, the cation polymerization system is poor in polymerizability as compared with the previously mentioned acid anhydrides and amines. Accordingly, this polymerizability is extensively studied. Examples of proposed methods include a method of increasing polymerizability by using oxetane resin in combination with epoxy resin according to Fine Chemical, Vol. 29, No. 19, 5, 2000, a method of using hydrogenated epoxy resin described in JP-A 2001-261780, a method of using cyclohexene oxide described in JP-A 2001-316450, and a system wherein vinyl ether is combined with a weak base as proposed in JP-A2000-109511. There is also an easier method which comprises adding a large amount of a catalyst for cation polymerization.
In the method of using oxetane resin, the rate of polymerization can be increased, but the initiation reaction itself should be coped by increasing the amount of a catalyst added, by simultaneously using epoxy resin, or by using higher energy, and there are problems such as deterioration of water resistance and coloration of resin caused by ionic components derived from the catalyst. In the method of using the hydrogenated epoxy resin having hydroxyl groups, there is a limit to use of epoxy resin having other skeletons used to exhibit functions. The method of using cyclohexene oxide has the problem of significant cure shrinkage. In the system where in vinyl ether is combined with a weak base, the weak base not forming a chemical bond with resin is added after curing, and thus there is the problem of inferior water resistance. It is found that when the amount of the catalyst for cation polymerization is increased, water resistance and corrosion resistance are significantly deteriorated.